1. Field of the Invention
The present invention relates generally to shielding structures for microwave circuits.
2. Description of the Related Art
Successful operation of microwave circuits is impossible unless significant isolation is provided to reduce interactions between sensitive circuit portions. Although electromagnetic shielding requires care at any circuit frequency, it generally becomes more difficult as the frequency of interest moves into the upper microwave region, e.g., past 10 GHz. In microwave circuits that are intended for high-volume production, the shielding structures should also be designed for manufacturability and low cost, i.e., they should be simple to form and assemble. Because space is often limited in microwave systems, the shielding structures should be compact and, preferably, provide structural rigidity to the microwave circuits which they shield.
A current shielding method places each sensitive circuit in its own metal enclosure and connects the enclosures with transmission lines, e.g., coaxial lines and waveguides. Although this method can achieve a high level of circuit isolation, its use is generally restricted to prototype and low-volume manufacturing because of the time-consuming fabrication details that it requires, e.g., machining of metal enclosures and covers to reduce minute gaps between them. In addition, the enclosures and their connecting transmission lines add significant volume to the shielded circuits.
In another current shielding technique, metal rings are arranged to surround each sensitive circuit. The rings extend between floor and ceiling of an enclosure. Signals are coupled between circuits with circuit-type transmission lines, e.g., microstrip and stripline, which are passed through small apertures in the rings. Although this shielding structure is generally less costly and more space efficient than separate circuit enclosures, the fabrication and installation of the metal rings is generally not compatible with high-volume production.